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Canon XI 50mm f0.75 Mount Help
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PostPosted: Fri Jan 05, 2018 2:47 pm    Post subject: Canon XI 50mm f0.75 Mount Help Reply with quote


I've been lurking here for a while, but this is my first post, so hello.

I've picked up a Canon 50mm f0.75 XI lens, which I believe was originally used in X-Ray applications. So far I've been using it on a Sony A7 body, with a DIY mount I've hammed together using some macro rings and rubber bands. It works fairly well, but I'd like to get something a bit more secure.

There are two different thread mounts on the back of the lens. It's difficult to get exact measurements as the rear element protrudes so much, but one is about 55mm, and one is about 76mm. The barrel of the lens itself also has marks that look like a clamp with screws could be used, and it's about 82mm across.

I was looking at getting a clamp similar to https://www.ebay.co.uk/itm/78mm-to-M65x1-thread-adapter-for-Super-Cinephor-2-152mm-lens/252927243601?hash=item3ae3a37551:g:xKgAAOSwC6dZrEVO

but wondered whether anybody had any more information on the XI lenses before I go down that road. It's proved almost impossible to find anything useful so far.

Thank you for any help!

PostPosted: Fri Jan 05, 2018 9:06 pm    Post subject: Reply with quote

At another forum, our member Woodrim posted a note about an M42 to Sony E-mount adapter he found on eBay that has a helical. So, I'm thinking that you might be able to McGuyver something together, especially if your lens's 55mm threads are the same as a lens or filter's 55mm threads. There might be a way to get from 55mm to 42mm (I'm thinking adapters used to reverse mount lenses). The helical might be handy because we don't know what the registration distance is for that lens. If it's less than M42, then, well, never mind. But if it's the same or greater, then an M42 adapter with helical might be just what you need.

To get an accurate measurement of the diameter of the threads on the rear of your lens, you really need a set of calipers. Even a compass will work. The type with the two pins work best. Come to think of it, the thing that looks like a compass you find in drafting kits but has two pins instead of one pin and a lead is called a caliper, isn't it? Anyway, you just transfer the measured diameter over to a ruler and there you go.

PostPosted: Fri Jan 05, 2018 9:50 pm    Post subject: Reply with quote

You can buy helicoids with various thread sizes:

Here's an example:


PostPosted: Fri Jan 05, 2018 11:56 pm    Post subject: Reply with quote

So... the problem with the Helicoids is that most of them start out with the lens too far away from the sensor for it to be useful for anything but macro shots. I can get it to a decent fixed focus length by using a combination of macro rings to gradually get to the right place, but fixing the lens to that length is the tricky part.

It's also tough to get affordable filter adaptors in the less common thread sizes without getting them from China. I don't mind doing that, but I've no way of being sure what the pitch size etc is, and don't want to buy things that take a month or two to arrive only to discover they don't work.

Getting M42 to Sony etc isn't a big deal obviously - just getting the lens itself into a usable mount.

I did some measurements by getting the circumference of the threads, then doing the math to find the diameter:

First thread:

Circumference: 180mm
Diameter: 57.3mm
Length (width): 5mm

Second thread:

Circumference: 200mm
Diameter: 63.6mm
Length (width): 4mm

Lens Barrel:

Circumference: 234mm
Diameter: 74.5mm
Length (width): 8mm
Screw bevel length from the front of the lens: 5mm
Screw bevel length from the rear of the lens: 8mm

Though using a compass is a great idea. I'll try that out.

I'm in talks with RAFcamera to get a mount that would work. Will report back if I have any success.

PostPosted: Sat Jan 06, 2018 12:44 am    Post subject: Reply with quote

What's the camera register distance for the lens?

PostPosted: Sat Jan 06, 2018 4:07 am    Post subject: Reply with quote

visualopsins wrote:
What's the camera register distance for the lens?

Not sure. That's why I'm posting here. Haven't been able to find any info out anywhere - so was hoping someone might be able to dig something up that I've missed!

PostPosted: Sat Jan 06, 2018 4:12 am    Post subject: Reply with quote